Jakubowski, M. (Corresponding author)FZJ*

2004
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Report No.: Juel-4143

Abstract: This thesis concerns the structures of the magnetic field induced by the Dynamic Ergodic Divertor (DED), which was recently installed at the TEXTOR tokamak in. Sixteen perturbation coils ergodize the field lines in the plasma edge and destroy the resonant surfaces. This creates an open chaotic system in the plasma edge. The structures of the magnetic field in the ergodic and the laminar region are systematically investigated using a set of codes, called “Atlas”. In Atlas, the field lines are traced using a mapping technique, which is based on the Hamiltonian formalism. This method is a fast and accurate algorithm to study the stochastic magnetic field lines. Typically, the ergodic region is a mixture of stochastic domains and island chains. The field lines in the stochastic domain have very long connection length and each field line fills the ergodic volume. After many turns around the torus they are deflected toward the divertor wall and leave the ergodic zone via so-called fingers. The field lines in the laminar zone are characterized by their very short connection lengths (compared to the Kolmogorov length) between two intersections with the wall. The structure of the flux tubes in the laminar zone is studied with Atlas. The flux tubes have the stagnation point half way between the intersections. When hitting the wall, they form stripe-like strike zones in front of the DED coils. At higher level of ergodization they split into pairs. In between these pairs, a private flux zones are established. It is shown, that the topology of the laminar zone resembles the structure of the scrape-off layer of the poloidal divertor. The detailed plasma properties (e.g. local density, temperature, flux amplitude and direction) are, however, complicated by the adjacent areas of flux tubes with different connection length. The ergodic and laminar structures in the plasma boundary depend strongly on the global plasma properties, in particular on the safety factor profile and the plasma pressure. These quantities determine the location and the separation of the resonant surfaces. The ergodization is systematically studied by varying the plasma current and poloidal beta. The width and structure of the ergodic and laminar region is a nonlinear function of the global parameters. As general tendency it has been found, that at the higher level of ergodization (i.e. at higher plasma current and lower beta poloidal) the laminar zone is dominant in the perturbed volume, while at lower level of ergodization the ergodic region dominates. Since the plasma pressure influences the pitch of the magnetic flux tubes in front of the DED coils (at constant edge safety factor), the resonance conditions of the flux surfaces vary as well. This leads to a systematic variation of the ergodization level as function of plasma pressure. A thermographic camera was set up and used to validate the predictions made with Atlas. The system measures temperature patterns on the divertor target plates. The heat flux density distribution is strongly non-homogenous, forming the expected stripe-like pattern. One observes four helical strike zones, which are parallel to the divertor coils. The measured patterns are in rather good agreement with the results from the modelling with the Atlas. The variation of the structure of the heat flux density deposition pattern with the plasma current and poloidal beta is measured. For increasing level of the ergodization the strike zone broadens and at some point splits up as it was predicted with Atlas. The predicted splitting of the divertor strike zones was clearly manifested. Imperfections of the alignment of the divertor target tiles were used to reveal local flux directions; namely, each of the power flux stripe consists of two parts with different direction of incoming heat and particle fluxes.

Keyword(s): magnetic field ; topology ; Textor Tokamak ; heat flux ; plasma diagnostics ; Tokamak device

1. Plasmaphysik (IEF-4)